1. Field of the Invention
The present invention relates to bypass type gas turbine engines, and more particularly, to a mixer arrangement for mixing the hot gas stream from the engine core with fan air from the bypass duct to thereby increase the temperature of the fan air prior to entering an afterburner section of the engine.
2. Description of the Related Art
In bypass type gas turbine engines, a portion of the air entering the turbine enters the core engine while the remaining portion of the air passes through a substantially annular duct surrounding the core engine. A low pressure fan is typically disposed upstream of the compressor of the core engine to pressurize the fan air entering the duct. Prior to entering a nozzle disposed aft of the core engine, a portion of the hot gas stream exhausting through the core is mixed with the fan air passing through the fan duct. During thrust augmentation or afterburning, liquid fuel is injected through spray bars where it is ignited with a mixture of the hot gas exhausting through the core and the fan air exhausting through the fan duct. This thrust augmentation, or afterburning, usually occurs in an afterburner section disposed immediately upstream of the nozzle and increases the energy of the exhaust stream to thereby increase the thrust of the nozzle.
A typical example of a mixer for mixing engine generated hot gases with fan air prior to discharge of the mixed gases through the engine nozzle is illustrated in U.S. Pat. No. 4,335,573. The mixer described therein is comprised of a plurality of alternating first and second chutes disposed around the core engine near the nozzle entrance. Each of the first and second chutes include an upstream portion and a downstream portion with the upstream portion being rotatably journaled on a bearing such that the first or second chutes may be rotated to achieve flow communication between the upstream and downstream portions of the first and second chutes. This rotation or indexing effects a temperature reduction of the flameholding elements in the downstream portions thereby minimizing infrared emissions from the engine nozzle. This reduction in infrared emissions aids in avoiding acquisition and tracking by heat seeking hostile missiles and permits countermeasures or evasive action to be taken against such missiles.
To the inventor's knowledge, all prior art mixer configurations, including that described in the above-referenced patent, exhibit a problem of blowout or resonance in the upper left-hand corner of the flight map where fan exit temperatures and afterburner pressures are low. In that portion of the flight map, afterburner operation is unavailable due to the possibility of blowout of the engine or damaging resonance vibration.
FIG. 1 illustrates a graph of a typical flight map wherein the abscissa corresponds to the mach number of the aircraft, and the ordinate corresponds to the pressure altitude in thousands of feet. In the graph of FIG. 1 the region of unstable combustion or blowout is depicted by cross-hatching in the area marked 200. In the past, marginally successful attempts have been made to alleviate the problem of blowout or resonance in portion 200 of the flight map of FIG. 1 by limiting the augmentation ratio of the engine. Additionally, some fuel distribution work in the afterburner section has shown some influence.